Nasal Leeches of Waterfowl
Causative Agent
  • A leech of the genus Theromyzon that feeds directly on blood from the nasal passages, trachea and mucous membranes of the eyes of migratory waterfowl.
  • Other species of leeches feed on other exposed surfaces of waterfowl.
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Nasal Leeches - Redhead Nasal Leeches - Mallard
Nasal leeches are often visible as they infest external nasal passages. Nasal leeches also infest the nasal sinuses.
  • Common on birds north of the 30th parallel and in western North America.
  • Peak infestations occur during the spring and summer months when leeches are actively seeking potential hosts and reproducing.
  • Winter slows the metabolic rate and activity of leeches.
  • Many aquatic birds are affected; commonly dabbling ducks (e.g., mallard, teal, wigeon, northern shoveler, etc.) and swans.
Signs and Symptoms
  • Leeches are 10-45 mm long when fully engorged, and are dark yellow to olive in color.
  • Free-living leeches are green with patterns of spots on the top surface.
  • Birds with leeches protruding from the nostrils or attached to the mucous membranes of the eyes are easily recognized at a distance with binoculars.
  • Leeches may become so engorged with blood that they resemble sacks of blood.
  • Infected birds may vigorously shake their heads, scratching at their bills with their feet, or sneeze in an attempt to dislodge leeches.
  • Leeches lodged in the nostrils and respiratory tract can cause labored breathing and gaping (breathing with an open bill) similar to that seen in birds affected by Aspergillus infection.
  • Feeding leeches can lead to extensive damage and inflammation of the lining of the nasal cavity.
  • Severe infestations of the eye can result in temporary blindness.
  • Leeches protruding from the nostrils or attached to the eyes can be removed with forceps.
Meat Edible?
  • Meat is edible but, if considerable blood-loss has occurred from leech infestation, the quality of meat may be decreased.
Human Health Concerns and Risk Reduction
  • Theromyzon leeches feed exclusively on birds and are not considered a threat to humans.
Samples for Diagnosis
  • Nasal leeches often cannot be observed externally, so submitting the entire carcass is warranted.
  • Nasal leeches will depart from a dead bird or may move to other areas of the bird, making them difficult to see. Leeches found on the carcass should be submitted as well for identification.
  • Leeches can be shipped live in pond water and maintained for several months in a refrigerator.
Similar Diseases
  • Similar breathing difficulty is observed in birds with aspergillosis
Further Reading
  • Tuggle B.N. 2001. Nasal leeches. Pp. 245-248 in Field Manual of Wildlife Diseases: General Field Procedures and Diseases of Birds. M. Friend, J.C. Franson (Tech. eds.), E.A. Ciganovich (ed.). Biological Resources Division Information and Technology Report 1999-001. U.S. Department of the Interior and U.S. Geological Survey. Washington, DC. (Chapter in PDF Format)
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